1. Field of the Invention
The present invention relates to a dehydration-sintering furnace for dehydrating and sintering a porous glass preform to obtain transparent glass.
2. Description of the Related Art
As illustrated in FIG. 8, a furnace 1 for dehydrating and sintering a porous glass preform includes a core tube 3, a heating body 5, a muffle tube 6, and a heat insulating member 7 as main components. The core tube 3 is made of quartz glass and the like and passes through a center portion of a furnace body 2 to accommodate therein a porous glass preform 4 supported by a supporting rod 10. The heating body 5 is arranged around the core tube 3 in the furnace body 2 to heat the porous glass preform 4 in the core tube 3. The muffle tube 6 is arranged to surround the outer periphery of the core tube 3 between the core tube 3 and the heating body 5. The heat insulating member 7 is arranged along an inner wall of the furnace body 2 at the outer side of the heating body 5. As another component, a gas supply port 8 is arranged at the lower part of the core tube 3. Gas, such as inert gas, necessary for dehydrating and sintering the porous glass preform 4 is supplied from the gas supply port 8. Furthermore, a gas exhaust tube 9 is arranged at the upper part of the core tube 3 to discharge the gas supplied from the gas supply port 8. Inert gas such as Ar and N2 is supplied into the furnace body 2.
The dehydration and sintering of a porous glass preform using such a dehydration-sintering furnace are carried out by relatively moving the porous glass preform in the up and down directions with respect to the heating body. For example, the dehydration and sintering may be carried out by moving the porous glass preform in the up and down directions with respect to the fixed heating body. Alternatively, the dehydration and sintering may be carried out by moving heating zones formed by a plurality of heating bodies in the up and down directions while fixing the porous glass preform. The movements of the heating zones are executed by switching the energization of the heating bodies.
With demands for optical fibers in recent years, larger and longer porous glass preforms for optical fiber are developed and thus larger furnaces for dehydrating and sintering the same to obtain glass are becoming necessary. However, a core tube is also becoming larger and larger with enlargement of the furnace, and thus the core tube made of quartz glass and the like may be deformed due to a buckling phenomenon caused by its own weight during heating.
Therefore, a method for providing collars at the outer periphery of a core tube and arranging muffle tubes above and below each collar to divide and bear the weight of the core tube in the longitudinal direction and to prevent the buckling deformation of the core tube has been known as disclosed in, for example, Japanese Patent Application Laid-open No. 2000-226217.
In recent years, the buckling deformation of the core tube cannot be prevented using such method due to further enlargement in the dehydration-sintering furnace.